[135.01] Discovery of an Extended Metal-Poor Stellar Halo in the Andromeda Spiral Galaxy

Understanding galaxy formation is a key goal of modern
cosmology. Our Galaxy's spheroidal halo of old stars halo
serves as a fossil record of its history of formation via
accretion of smaller galaxies. Studies of our neighbour, the
Andromeda galaxy, have concluded that its spheroid contains
chemically enriched (``metal-rich'') stars out to a radius
of 30 kpc with an exponential fall-off in density and thus
resembles a galactic ``bulge''. Were the true halo of
Andromeda to be found, our detailed yet global view of its
stellar dynamics, substructure, chemical abundance, and age
distribution would constrain hierarchical halo formation
models in a direct new way. We report here on the discovery
of this hitherto elusive component: a halo of metal-poor
Andromeda stars, distinct from its bulge, with a power-law
brightness profile extending beyond R=160 kpc. This is 3-5
times larger than any previously mapped Andromeda
spheroidal/disk component. The Milky Way and Andromeda halo
radii together span more than one-third of the distance
between them. This suggests that stars occupy a substantial
volume fraction of our Local Group, and possibly all galaxy
groups.

P.G. acknowledges support from NSF grant AST-0307966. Data
presented herein were obtained at the W.M. Keck Observatory,
which is operated as a scientific partnership among Caltech,
the Univ of California and NASA. The Observatory was made
possible by the generous financial support of the W.M. Keck
Foundation.